1. Field of the Invention
The present invention relates to the introduction of waste materials into rotating devices such as cement kilns in general and more particular to an improved method and apparatus for continuous introduction and burning of hazardous waste in rotary, cement kilns of the dry type.
2. General Background
The apparatus and feeding system disclosed herein are applicable to situations where a need exists to feed waste materials into a rotating cylinder at or near its stationary, material, input end in a continuous and controlled manner. The application of the feeding system is not, however, restricted to the feeding of fuel grade materials alone. The system is capable of metering raw material or chemical additives into rotating cylinders from a stationary location. Kiln dust, fly ash and many other raw materials can be beneficially utilized by the continuous introduction of such materials directly into the calcining zone of a cement kiln. Horizontal, rotary reactors and mixers utilized in the chemical industry can use the feeding system to introduce chemical additives at strategic steps in the reaction process.
The range of applications for the improved, rotary kiln, feeding apparatus disclosed herein is quite broad. The preferred embodiment disclosed, however, is specifically designed to be utilized by cement kilns and aggregate kilns in the feeding and burning of waste material as a means of thermal destruction as well as energy recovery.
There are two types of cement kilns, one wet the other dry. The present invention is related to dry type kilns and more particularly to preheat kilns because the calcining zone is located at or near the mineral's point of entry. Dry kilns occur in many forms. Some dry kilns operate in much the same way as wet kilns except the minerals are introduced in dry powder form. Long dry kilns of this type are more energy efficient than wet kilns for obvious reasons. The newer more efficient dry kilns employ a vertical preheating tower prior to entry of the minerals into the kiln itself. Hot combustion gases flow from the kiln upwards through the preheat tower while dry powdered minerals cascade downward. The countercurrent flow of combustion gases preheats the minerals prior to entry into the kiln, and for this reason, preheater kilns are usually much shorter in length, usually in the range of 150 to 250 feet. A further improvement in preheater technology includes the introduction of hot combustion gases from a separate burner mounted in the first stage of the preheat tower. With this arrangement, calcining of the minerals begins in the preheater tower, and it finishes in the kiln. Precalcining kilns represent the latest technology to date and are the most energy efficient. Such kilns produce a calcining zone in excess of the required 1800 degrees F. to produce complete thermal destruction and are accessible from a stationary point at the riser duct connecting the kiln to the preheater tower. It is for the above reasons that such dry type, cement kilns function as excellent incinerators. Therefore, the concept of burning hazardous waste is becoming more and more economically viable. The role of the cement industry in the thermal destruction of liquified hazardous waste has long been recognized. However, the present challenge in this industry is to burn organic sludge and solids that heretofore have been exclusively in the domain of the incinerators and landfills. It is to this end that the invention is addressed.
3. Discussion of the Related Prior Art
A good summary of the state of the industry with respect to the use of hazardous waste for fueling cement kilns is the article entitled "Cement Kilns 1990" published in Environmental Information Digest, June 1990, pages 14 to 23.
The closest relevant prior art is encompassed in U.S. Pat. Nos. 4,850,290; 4,930,965; 4,969,407; 5,224,433; 5,083,516 and 5,257,586. Most of these patents teach a single method and apparatus for the charging of containerized solid fuel, in the form of hazardous waste, into rotary kilns. One such embodiment utilizes the charging process, effecting entry into the calcining zone of a dry kiln through a port in the riser duct. Such charging is dependent on the opening and closing of a valve system to prevent escaping gases. The principle of the containerized hazardous waste depends heavily on the assumption that such waste must be blended to form hazardous waste homogenate prior to containerization, and that such containerization is essential to prevent overloading of the kiln gas stream with volatile organic matter. It has also been found that additional oxygenation is required in most cases where such charging is used, and in particularly in cases where improper blends of the waste homogenate is common. Only U.S. Pat. No. 5,083,516 teaches a continuous feed system utilizing a screw feeder and transfer means. However, this system is limited to use with infeed through commutation into the rotating portion of a kiln.
There are several problems associated with the prior art that has deterred its widespread use. First, the prior art technology requires that the waste be shredded and packed into six gallon containers. This requirement causes waste processors to spend large sums of money for materials and labor to containerize such materials and further needlessly exposes personnel to the hazardous materials. The process is slow and labor intense and has proven to be impractical to containerize large quantities of material. The intermittent introduction of the small containers results in irregular releases of energy and emissions thereby threatening the stable conditions within the kiln. The consistency of fuel quality varies from container to container regardless of how careful the mixer may be. Finally, the prior art technology requires the kiln to intake excess air at the point of entry of the waste materials, when supplied from the stationary end of a dry kiln, in an attempt to ensure adequate oxygen in the calcining zone to promote complete combustion of the solid fuel thus reducing the overall thermal efficiency of the kiln. It has been established that excess combustion air, introduced intermittently at the stationary inlet end of a rotary kiln, has a detrimental effect on the quality of the clinker (product material) produced by a cement kiln. It is therefore essential that combustion air be controlled and introduced directly into the calcining zone.
Several methods of burning waste materials in cement kilns are well known in the industry. They include: burning dispersed liquids in the primary burner, burning pneumatically conveyed solids in the primary burner, and finally, burning material injected into the calcining zone through the use of containerized material as discussed above. In each case it is essential to maintain primary burner flame temperatures in the 3,500 degree F. range in order to form quality clinkers (which must reach approximately 2650 degrees F. to properly transform). It is also essential that a heat value in the fuel be maintained at a minimum of 10,000 BTU's/Lb. in order to maintain the required flame temperature. This means that low BTU value solid waste becomes increasingly difficult to utilize in the primary burner. To do so requires that such waste material be blended with high BTU value material to yield the target value of 10,000 BTU's/Lb. The same problem exists when pneumatically conveyed solids are feed into the primary burner. Solid hazardous waste as a family tend to have low BTU values of between 5,000 and 7,500 BTU/LB, thus such materials are not ideally suited for burning in the primary burner, where fuel quality is a critical factor. In addition solids have a higher concentration of inorganic constituents than liquids; therefore, when they are burned in the primary burner such inorganic constituents (ash) tend to follow the combustion gas flow, down the center of the kiln and out into the kiln dust. As a result, there is often an unacceptable build up of inorganic contaminants in the kiln dust. The present invention has overcome the shortcomings of the prevailing technology in several significant ways. First, the present system provides for the direct feeding of combustion air and materials into the calcining zone, where the temperature is sufficiently elevated, usually between 950 degrees C. and 1200 degrees C., to provide thermal destruction of all organic constituents, without the need for containerization and off-site blending. It is a known fact that fuel grade material can be burned within the mineral bed of the calcining zone, resulting in a more efficient method of heat transfer to the minerals. It is, therefore, possible to use relatively low BTU value materials to enhance the temperature within the calcining zone. Moreover, it has been found that the mineral bed passing through the calcining zone, of a rotating cement kiln, becomes chemically reactive due to the dissociation of carbon dioxide from the limestone minerals used in manufacturing cement. As a result the mineral bed is at its most active at this location. The inorganic constituents contained in waste materials introduced directly into the calcining zone, chemically bond to the minerals passing through the calcining zone. As a result the low BTU value materials can be beneficially utilized as a fuel in the calcining zone without jeopardizing cement and kiln dust quality. Since solids tend to have high concentrations of inorganic material, the calcining zone is better suited for burning such materials than the primary burner. The mineral bed is not reactive outside the calcining zone near the primary burner; therefore, inorganics which fall in this area are not chemically bound into the matrix. As a result, burning solid materials in the primary burner may be subject to limitations dictated by the deductibility of toxic constituents in the finished cement and kiln dust. Prevailing technology has shown that as much as 40% of a kiln's fuel demand can be satisfactorily burned in the calcining zone. The apparatus and methods to be presented herein represent improvements in the prevailing technology by demonstrating that waste materials can be burned most effectively in a cement kiln's calcining zone by continuously feeding material in a completely controlled manner without the need for prior containerization of the material and further that the introduction of additional combustion air into the calcining zone allows for an improvement in the quantity of fuel that can be burned in the zone as compared to the current technology. In addition, methods and ancillary equipment are presented which allow for receiving and processing bulk quantities of waste materials, directly from the generators, prior to simultaneously feeding from two or more sources to the kiln input.